The present invention relates generally to the field of continuous emissions monitoring systems (CEMS), and particularly to the validation of sensor readings in replacement of sensor values for calculations of emissions in such systems.
Emissions monitoring systems are common and necessary components of modern industrial and other plants, particularly manufacturing plants, power generating plants, and so forth. For almost 20 years software CEMS have been successfully deployed to detect and predict emissions in such industrial plants, including emissions of boilers, furnaces, gas turbines, thermal oxidizers and other combustion sources, in the United States and around the world. One such successful software CEMS is commercially available from Rockwell Software, a division of Rockwell Automation, Inc. of Milwaukee, Wis. under the commercial designation “software CEM.” Such systems allow for progressive software-based solutions for emissions compliance requirements. In conjunction with other products, they may afford environmental compliance and reporting, providing “active compliance” continuously and in real-time as a predictive emissions monitoring system (PEMS), increasing product integrity and optimizing energy efficiency.
In certain CEMS, sensor data is generated that is representative of the emission of certain components of interest, typically in gaseous form, although other forms may be detected. The systems may allow for monitoring and modeling of such emissions, such that if an emissions sensor is non-communicative or produces a clearly erroneous result, a modeled value may serve as a surrogate for the sensed value. Models may also be developed for the inputs to the emissions model. Such functionality would typically be provided in a PEMS architecture.
Most recently, in 2009 the U.S. Environment Protection Agency finalized new standards for the acceptability of certain classes of PEMS when they are initially installed. The standard, now known as Performance Specification 16 (PS-16), includes a number of refined requirements, including one relating to a Sensor Evaluation System, in Section 6.1.8 of the Standards. The subject section states that a compliant PEMS must be designed to perform automatic or manual determination of defective sensors on at least a daily basis. Moreover, Section 6.1.2 of the Standards states a requirement that a compliant PEMS must operate within the range of minimum and maximum values (an “operating envelope”) of the data from the PEMS development process. In general, the PEMS development process is the process used to develop the predictive model. In many cases these values constitute the values for a training dataset used for emissions model input variables.
In known PEMS products with sensor validation methods input values to the emissions model that fall outside of an operating envelope may be used as an indication of sensor drift. In such cases, conventional systems may substitute a reconstructed value for the sensor value, which by definition would fall within the operating envelope, the substituted value being based upon the model training dataset. However, such prior techniques may not be fully consistent with the intent of PS-16 insomuch as they allow sensor readings that fall outside the operating envelope to be replaced with reconstructed values that are within the envelope.
There is a need, therefore, for improved CEM systems and techniques that can respond to the evolving requirements, particularly regarding the replacement of modeled or reconstructed values for sensed values as inputs to an emissions model.